Storm water estimation by rational method

The rational method can be precisely applied to areas less than 50 hectares. For large areas, empirical formulae are used to determine peak rate of runoff
According to the rational method:

• If rainfall is applied to an impervious surface at a constant rate, the resultant runoff from the surface would finally reach a rate equal to the rainfall.
• In the beginning, only a certain amount of water will reach the outlet, but after some time, the water will start reaching the outlet from the entire area, and the runoff rate would equal to the rate of  rainfall
• The period after which the entire area will start contributing to the runoff is called the time of concentration.
• It is the time required by the water to reach the outlet from the most remote point of the drainage area.
• The runoff resulting from a rain having a duration lesser than the time of concentration will not be maximum, as the entire area will start contributing to runoff in this case.

Based upon this principles, the rational formula was developed by the efforts of kuichling of America, Fruhling of Germany and Lloyd davis of England.
This formula states that,

Qp= 1/36*K*Pc*A

Where,
Qp= peak rate of runoff in cumecs
K= Coefficient of runoff
Pc= critical rainfall intensity during the critical rain fall direction
A= Catchment area contributing to runoff at the considered point in hectare

Coefficient of runoff is in fact, the impervious factor of runoff, representing, the ratio of runoff to precipitation.
The value of K increases as the imperviousness of the area increases.

For perfectly impervious area K=1.

For the design of drains having catchment area larger than 400 hectares, it is generally advisable to use the empirical formula suggested for that region.
Various empirical formulae for calculating storm water run-off have been suggested by various investigators. Some if the important formulae are given below:

• Burki zielgler formula
• Dicken’s formula
• Ryve’s formula
• Inglis formula